ANTICORROSION ADHESIVE COMPOSITION AND METHOD

Abstract: An adhesive composition comprising particles of an inorganic oxide of silica or alumina having cations bound thereto, which particles inhibit or prevent corrosion of metal when the cured adhesive composition is in contact therewith. A method of bonding is also described.

Description: Antlcorrosion adhesive composition and method FIELD OF THE INVENTION This invention relates to adhesives containing an agent to resist corrosion on metal surfaces. BACKGROUND OF THE INVENTION Corrosion resistant treatments for metals are well known in the art and play an important role in maintaining the bond between a coating, such as paint, or an adhesive to the metal over long periods of time, especially in corrosive environments. The most commonly encountered corrosive environments include exposure to salt water from oceans, road treatments for ice containing corrosive chemicals and/or salt, acid rain, and the like. There are methods of treating metal surfaces before applying paints and adhesives to protect the metal surface from corrosion. The methods generally involve a conversion coating in which a protective oxide is formed on the metal surface, or an etching process to form a protective oxide. The etching process may further include processes of anodizing and/or priming. One well known method of preparing aluminum is referred to as an "FPL Etch" which has been used in the aerospace industry since the early 1950's. The FPL etch process includes the steps of optionally degreasing the aluminum part with solvent, degreasing with an alkaline solution at 180°F, immersing the part in an aqueous solution of sulfuric acid and sodium dichromate at 160°F, rinsing, and drying the part at room temperature and elevated temperature. Although the process has been useful, there are environmental and health concerns in using chromates and corrosive acids. Conversion coatings known in the art include phosphate conversion coatings, chromate conversion coatings, and cobalt conversion coatings. Other methods of improving corrosion resistance include adding a corrosion inhibitor to adhesives. Inhibitors currently used include chromate salts, such as barium chromate, strontium chromate, magnesium chromate, etc. Certain cations, such as zinc and calcium, have been used as corrosion inhibitors in protective coatings such as paints. The cations are typically used in the form of sparingly water soluble salts. Shieldex™ anti-corrosion pigment is an ion-exchanged silica said to be useful in paints and coatings. SUMMARY OF THE INVENTION The present invention provides a curable, structural epoxy adhesive composition comprising: (a) an epoxy resin having an average epoxide functionality of greater than one; (b) a base curing agent in an amount sufficient to cure the epoxy resin through reaction of a nucleophilic or electrophilic group contained in the base curing agent with the epoxy ring contained in the epoxy resin; (c) particles comprising an inorganic oxide of silica or alumina having cations bound thereto, the particles being of a type which and present in an amount effective to inhibit or prevent corrosion of metal to which the epoxy adhesive composition has been applied. The present invention further provides a method for inhibiting or preventing corrosion of a metal substrate which has been bonded to another substrate, the method comprising including particles comprising an inorganic oxide of silica or alumina having cations bound thereto in the adhesive composition being employed to bond the metal substrate to the other substrate, the particles being of a type which and present in the adhesive composition in an amount effective to inhibit or prevent corrosion of the metal substrate in the area thereof which is in contact with the adhesive composition. DESCRIPTION OF THE INVENTION The present invention provides epoxy adhesive compositions that are useful as structural adhesives that provide corrosion resistance to the substrate from salt solutions. The adhesives are particularly useful for metal substrates such as aluminum and steel and the preferred compositions do not contain undesirable heavy metal elements such as chromium. Structural adhesives form strong integral bonds between substrates. Bonds formed with structural adhesives have a room temperature bond strength, as measured by a test well known in the industry, referred to as a T-peel test, of at least 10 pounds per lineal inch (pli) on a bond line thickness of 0.010 inch (0.25mm). The upper limit on the structural bond strength would be the cohesive failure of the substrate or yielding of the substrate. The adhesive compositions of the present invention form room temperature T-peel bond strengths on a bond line thickness of 0.010 inch (.254 mm) of at least 12 pli, with preferred compositions having bond strengths greater than about 14 pli, and the most preferred compositions having bond strength greater than 17 pli. Structural adhesive bonds can also be characterized by a room temperature modulus, as measured by a test known in the industry as the overlap shear test, of at least 3 megaPascals (MPa). The upper limit of the overlap shear strength would be the cohesive failure of the substrate, or yielding of the substrate. The adhesive compositions of the invention form bonds with overlap shear strengths at room temperature of at least 5 MPa, with preferred compositions forming overlap shear strength bonds of at least 7 mPa, and with the more preferred compositions forming overlap shear strength bonds of at least 10 MPa, and with the most preferred compositions forming overlap shear strength bonds of at least 14 MPa. Structural bonds typically have a thickness greater than 2 mils (0.5 mm). In the practice of the invention, bond lines formed are typically greater than 5 mils (0.127 mm). Epoxides that are useful in the adhesive composition of the present invention can be any organic compound having at least one epoxy ring that is polymerizable by ring opening. Preferred are organic compounds having an average epoxy functionality greater than one, and preferably at least two. The epoxides can be monomeric or polymeric, and aliphatic, cycloaliphatic, heterocyclic, aromatic or mixtures thereof. The more preferred epoxides are aromatic and contain more than 1.5 epoxy groups per molecule and most preferably more than 2 epoxy groups per molecule. The useful materials have a molecular weight of about 150 to 10,000 and preferably from about 300 to 1,000. Useful materials include linear polymeric epoxides having terminal epoxy groups (e.g., a diglycidyl ether of a polyoxyalkylene glycol), polymeric epoxides having skeletal epoxy groups (e.g.,polybutadiene polyepoxy), and polymeric epoxides having pendant epoxy groups (e.g., a glycidyl methacrylate polymer or copolymer), and mixtures thereof. Useful epoxide containing materials include compounds having the required molecular weight of the general Formula I: wherein: R' is alkyl, alkyl ether, or aryl, preferably aryl and n is an integer between 2 and 6. Preferred are aromatic glycidyl ethers such as those prepared by reacting a polyhydric phenol with an excess of epichlorohydrin. Examples of useful phenols include resorcinol, catechol, hydroquinone, and the polynuclear phenols including p,p'-dihydroxydibenzyl, p,p'-dihydroxydiphenyl, p,p'-dihydroxydiphenyl sulfone, p,p'-dihydroxybenzophenone, 2,2'-dihydroxy- 1,1-dinaphthylmethane, and the 2,2', 2,3', 2,4', 3,3', 3,4', and 4,4' isomers of dihydroxydiphenylmethane, dihydroxydiphenyldimethylmethane, dihydroxydiphenylethylmethylmethane, dihydroxydiphenylmethylpropylmethane, dihydroxydiphenylethylphenylmethane, dihydroxydiphenylpropylphenylmethane, dihydroxydiphenylbutylphenylmethane, dihydroxydiphenyltolylmethane, dihydroxydiphenyltolylmethylmethane, dihydroxydiphenyldicyclohexylmethane, and dihydroxydiphenylcyclohexane. Also preferred are polyhydric phenolic formaldehyde condensation products as well as polyglycidyl ethers that contain as reactive groups only epoxy groups or hydroxy groups. Compounds of the above general Formula I, but wherein n = l, are useful as optional additives in the composition of the instant invention so long as, in the preferred embodiment, the average epoxy functionality relative to the total number of epoxy compounds employed is greater than one. Useful materials include diglycidyl ethers of bisphenol A and of novolak resins, such as described in "Handbook of Epoxy Resins" by Lee and Nevill, McGraw-Hill Book Co., New York (1967), incorporated herein by reference. Epoxides with fiexibilized backbones are also useful. Preferred materials include diglycidyl ethers of bisphenol A and diglycidyl ethers of bisphenol F, and most preferably diglycidyl ethers of bisphenol A, because of the desirable structural adhesive properties that these materials attain upon curing. Examples of commercially available epoxides useful in the invention include diglycidyl ethers of bisphenol A (e.g., those available under the trademarks Epon 828, Epon 1001, and Epon 1510 from Shell Chemical Co., and DER-331, DER-332, and DER-334 available from Dow Chemical Co.); diglycidyl ethers of bisphenol F (e.g., Epiclon™830 available from Dai Nippon Ink and Chemicals Inc.); silicone resins containing diglycidyl epoxy functionality; flame retardant epoxy resins (e.g., DER 580, a brominated bisphenol type epoxy resins available from Dow Chemical Co.); and 1,4- butanediol diglycidyl ethers. In the practice of the invention, a base curing agent is used in an amount sufficient to cure the epoxy adhesive composition. The amount can vary from an approximate stoichiometric amount based on the type of epoxy resin used to an excess of either the epoxy or the base curative, depending upon the end use of the epoxy adhesive. The amount typically ranges from about 1.5 to 200 parts by weight of curing agent per 100 parts of the total amount of epoxide used. Preferably, the base curing agent will be present in an amount of about 2.5 to 75 parts by weight of the curing agent per 100 parts of epoxide. The base curing agent contains at least one nucleophilic or electrophilic group which reacts with the epoxy ring to cross-link the adhesive composition. Suitable base curing agents include polyamide resins, aliphatic amines, polyether diamines, aromatic amines, polyamines, polyamidoamines, polyetherdiamines, phenol compounds, and mercaptan resins. Examples of primary amines include di-(4-aminophenyl)sulfone, di-(4-aminophenyl)-ethers, and 2,2-bis(4-(aminophenyl)propane, ethylene diamine, hexamethylene diamine, isomers of hexamethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, bishexamethylene triamine, N,N'-Bis (3-aminopropyl)- 1,2-ethane diamine, N-(3-Aminopropyl)-l,3-propane diamine N-(2-aminoethyl)- 1,3 propane diamine, isomers of cyclohexane diamine, 4, 4 '-methylene biscyclohexanamine, 4, 4 '-methylene bis[2-methylcyclohexanamine] , isophorone diamine. Examples of useful tertiary amines are dimethylaminopropylamine and pyridine. Examples of useful aromatic amines include di-(4-aminophenyl)sulfone, di-(4-aminophenyl)ether, 2 ,2-bis(4-aminophenyl)propane, 4,4' -diamino diphenylmethane, 3,3'-dimethyl(4,4'-diaminodiphenyl methane, m-phenylene diamine, p-phenylene diamine, m-xylylene diamine, toluene diamine, 4,4'- methylene dianiline benzidine, 4,4'-thiodianiline, 4-methoxy-l,3-phenyldiamine, 2,6-diaminopyridine, and dianisidine. Examples of polyether diamines include 4,9-dioxadodecane-l,12- diamine, 4,7,10-Trioxatridecane-l,12-diamine, bis(3-amino propyl)polytetrahydrofurans of varying molecular weights, and commercially available from Texaco Chemical Co. under the Jeffamine trade name as D230, D400, D2000 and T403. Suitable polyamido amines are the reaction products of polyamines and dimer acids. Dimer acids are prepared by dimerizing C lg or C 22 fatty acids from vegetable oils or animal fats. The dimer acids are then reacted further with polyamines by a condensation reaction to produce the polyamido amine oligomers. These oligomer are described by V. Brytus, Modern Paint and Coatings. Vol. 74 No.10, p. 172 (1984). Examples of phenol compounds include phenol, substituted alkyl phenols (nonyl phenol), diphenols such as catechol, and alkyl substituted catechol, resorcinol, hydroquinone. Examples of mercaptan resins include alkyl dimercaptans such as ethane dithiol, nonane dithiol, penta erythritol tetra (3-mercapto propionate), trimethylol propane tri(3-mercapto propionate), glycol dimercapto acetate, thiol terminated polyethers and thiol terminated poly sulphides. Also useful are boron complexes, and in particular, boron complexes with monoethanolamine; imidazoles such as 2-ethyl-4-methyl imidazole; guanidines such as tetramethyl guanidine; substituted ureas such as toluene diisocyante urea; dicyandiamide; and acid anhydrides such as 4- methyltetrahydroxyphthalic acid anhydride, 3-methyltetrehydroxyphthalic acid anhydride, and methylnorbornenephthalic acid anhydride. Mixtures of more than one curative may be used. Preferred curatives for one-part adhesive compositions are amines, acid anhydrides, guanidines, dicyandiamide, and mixtures thereof. Specific examples of base curing agents are Ancamide™Series, commercially available from Air products and Chemical Company, and the Scherex Series, commercially available from Schering-Berling. Accelerators known in the art can also be added to increase the cure rate of the epoxy adhesive. Such accelerators include compounds that can act as a curative when used alone, but when combined with a different class of curatives, will accelerate the curing of the epoxy adhesive composition. Examples of useful accelerators include phenolic compounds, tertiary amines, dicyandiamides, imidazole, substituted imidazole hexakis imidazole nickel phthalate complex, substituted ureas and calcium trifluoromethylsulfonate. These accelerators may be used alone or in combination together to accelerate the cure of an epoxy adhesive combination. Some examples of useful combinations include phenolic compounds with tertiary amines, dicyandiamides with imidazole and/or substituted imidazoles, dicyandiamides with substituted ureas, dicyandiamides with hexakis imidazole nickel phthalate complex and calcium trifluoromethyl sulphonate with imidazoles. A preferred curing agent/accelerator combination is toluene diisocyanate urea and dicyandiamide. The preferred amount of the accelerator is from about 0.5 to 15 percent by weight of the adhesive system. The epoxy adhesive composition includes a particulate ion exchange corrosion inhibiting additive. The additive particles are formed from an inorganic oxide of silica or alumina and have cations chemically bound to them that are useful for corrosion inhibition. The useful cations include calcium (Ca 2+ ), zinc (Zn 2+ ), cobalt (CO 2"1" ), lead (Pb 2+ ), strontium (Si 2"1" ), lithium (Li + ), barium (Ba 2+ ), and magnesium (Mg 2+ ). Preferred cations include calcium and zinc. The additive particles will preferably have an average diameter of about 0.1 to 200 microns. More preferably, the particles have an average diameter of about 1 to 50 microns. Suitable additive particles include a calcium ion exchanged amorphous silica gel commercially available from W. R. Grace & Co. under the tradename "Shieldex". The epoxy adhesive composition preferably includes a toughening agent, and in particular, a polymeric toughening agent or a combination of polymeric toughening agents. Useful toughening agents have an epoxide incompatible component substantially insoluble in the epoxy resin and an epoxide compatible component substantially soluble in epoxy resin. The toughening agents which are useful in the present invention include polymeric compounds having both a rubbery phase and a thermoplastic phase, such as graft copolymers having a polymerized diene rubbery core and a polyacrylate or polymethacrylate shell; graft copolymers having a rubbery core with a polyacrylate or polymethacrylate shell; and elastomeric particles polymerized in situ in the epoxide from free-radical polymerizable monomers and a copolymeric stabilizer. Specific examples of useful toughening agents include graft copolymers having a polymerized diene rubbery backbone or core which is grafted to a shell of an acrylic acid ester or methacrylic acid ester, monovinyl aromatic hydrocarbon, or a mixture thereof, such as disclosed in U.S. Patent No.3,496,250. Preferable rubbery backbones comprise polymerized butadiene or a polymerized mixture of butadiene and styrene. Preferable shells comprising polymerized methacrylic acid esters are lower alkyl (C j -C^ substituted methacrylates. Preferable monovinyl aromatic hydrocarbons are styrene, alpha-methylstyrene, vinyltoluene, vinylxylene, ethylvinylbenzene, isopropylstyrene, chlorostyrene, dichlorostyrene, and ethylchlorostyrene. Further examples of useful toughening agents are acrylate core-shell graft copolymers wherein the core or backbone is a polyacrylate polymer having a glass transition temperature (T g ) below about 0°C, such as polybutyl acrylate or polyisooctyl acrylate to which is grafted a polymer (shell) having a T g above 25 °C such as polymethylmethacrylate. Still further examples of toughening agents useful in the invention are elastomeric particles that have a T below about 25 °C and have been polymerized in situ in the epoxide before mixing with the other components of the composition. These elastomeric particles, commonly referred to as "organosols", are polymerized from free-radical polymerizable monomers and a copolymerizable polymeric stabilizer that is soluble in the epoxide. The free- radical polymerizable monomers are ethylenically unsaturated monomers or diisocyanates combined with co-reactive difunctional hydrogen compounds such as diols, diamines, and alkanolamines. Examples of these elastomeric particles are disclosed in U.S. Patent No. 4,525,181. Still other toughening agents are rubber modified liquid epoxy resins. An example of such a resin is Kraton™ RP6565 Rubber available from Shell Chemical Company. The modified epoxy resin is made from 85% by weight Epon™ 828 and 15% by weight of a Kraton™ rubber. The Kraton™ rubbers are known in the industry as elastomeric block copolymers. Toughening agents can also include liquid epoxies, liquid amines, polyether diamines, polyhydroxyethers, polyvinylacetals, and liquid acrylonitrile butadiene polymers, butadiene/nitrile rubbers, carboxylated butadiene/nitrile rubbers, amine-terminated butadiene/nitrile rubbers, carboxyl-terminated butadiene/nitrile rubbers and the amine or carboxyl terminated adducts of the polymers with epoxy resins. Amine-terminated and carboxyl-terminated butadiene-acrylonitrile rubbers are commercially available from B.F. Goodrich under the HYCAR tradename as ATBN and CTBN reactive liquid polymers. Combinations of toughening agents may also be used to enhance the properties of the cured epoxy adhesive. The toughening agent is preferably used in an amount equal to about 3 to 35 parts by weight, and more preferably about 5 to 15 parts by weight per 100 parts by weight of the epoxy resin. The toughening agents of the present invention add strength to the composition after curing without interfering with curing. The toughening agent may or may not react with the epoxide. In some cases reactive diluents may be added to control the flow characteristics of the adhesive composition. Suitable diluents have at least one reactive terminal end portion and preferably, a saturated or unsaturated cyclic backbone. Preferred reactive terminal ether portions include glycidyl ether and vinyl ether. Examples of suitable diluents include the diglycidyl ether of resorcinol, diglycidyl ether of cyclohexane dimethanol, diglycidyl ether of neopentyl glycol, triglycidyl ether of trimethylolpropane dipentene, and the divinyl ether of cyclohexanedimethanol. Commercially available reactive diluents are "WC-68" from Rhone Poulenc, and Rapicure™ CHVE, a divinyl ether of cyclohexanedimethanol available from Allied-Signal Corp. of Morristown, NJ. Various other adjuvants can be added to the epoxide composition to enhance properties of the composition before and after curing. Included among useful adjuvants are nonreactive diluents; plasticizers such as conventional phosphates and phthalates; flame retardants such as borates, metaborates, aluminum hydroxide, magnesium hydroxide, and bromine compounds; thixotropic agents such as fumed silica to provide flow control; pigments to enhance color tones such as ferric oxide, brick dust, carbon black, and titanium dioxide; fillers such as talc, silica, magnesium, calcium sulfate, beryllium aluminum silicate; clays such as bentonite; glass and ceramic beads and bubbles; compounds imparting X-ray opacity, such as barium metaborate; and reinforcing materials, such as woven and nonwoven webs of organic and inorganic fibers such as polyester, polyimide, glass fibers, and ceramic fibers. Dispersing agents and wetting agents, such as silanes, can also be added so long as they do not interfere with the curing reaction of the epoxy adhesive composition. The adjuvants can be added in an amount effective for the intended purpose; typically, amounts up to about 50 parts of adjuvant per total weight of formulation can be used. Presently preferred compositions of the invention or compositions to be used in the methods of the invention are substantially free of conventional corrosion inhibitors such as aluminum phosphates and chromate salts. Further, preferred methods of the invention rely on the use of an oxide of silica or alumina having cations bound thereto, and do not involve the use of other corrosion inhibitors such as aluminum phosphates or chromate salts either in the adhesive composition or as a pretreatment of one or both of the surfaces to be bonded before application of the adhesive. The epoxy adhesive composition of the present invention may be formulated in a variety of ways, including one-part and two-part adhesive systems. By providing a two-part composition, with the two parts being combined prior to use of the composition, desirable shelf-life or pot-life of the composition is obtained. In some applications, it is desirable to select the amounts and the distribution of the ingredients in each part to provide viscosity control and better mixing of the two parts. For example, the fillers can be divided so that each part contains a portion of the fillers used. The epoxy compositions of the present invention can be cured by any means which allow sufficient heat to start the curing reaction. The means of curing can include conventional ovens, induction heating, infrared radiation, microwave radiation, immersion into liquid baths, or any combination thereof. For two part adhesive compositions, the curing can be effected at room temperature for about 24 hours. Typically, the final curing is conducted at a temperature in the range of about 15°C to about 230°C for a time ranging from about 1 second to about 2 hours. Curing may be done in several stages, e.g. , induction curing for 30 seconds, and oven curing at 215 °C. The curing time will depend upon the particular process for curing. Induction heating times typically range from about 1-60 seconds while oven curing times can range from about 0.1 to about 2 hours. The epoxy adhesive compositions of the present invention are especially useful for bonding metal to metal and plastic to metal, although it can be used for bonding only plastic surfaces. Examples of metal surfaces include steel, titanium, oily steel, aluminum, and magnesium. Plastic surfaces include sheet molding compounds, polyethylene, polycarbonate, polyester, polyurethane, acrylonitrile butadiene styrene, and ureaformaldehyde. The epoxy adhesive can be used in assembling parts such as for automobiles, aircraft, refrigeration units, etc. The following non-limiting examples serve to further illustrate the present invention in greater detail. TEST PROCEDURES LAP SHEAR STRENGTH This test measures the shear strength that an epoxy adhesive composition will achieve in a single overlap bond after being fully cured. The lap shear strength is also referred to as the "overlap" shear strength. A test sample is prepared by applying the adhesive to 2.54 cm x 10.16 cm overlapping aluminum strips and curing as detailed below. The aluminum strips used in the tests were: A - 1.6 mm thick 6111 aluminum having a "mill finish", available from Alcoa Aluminum Co. B - 1.6 mm thick 5754-0 treated aluminum from Alcan. C - 0.9 mm thick 6111 aluminum treated with Parker MP404 Lube (treatment available from Parker Div. of Henkel Corp.) D - 0.9 mm thick 6111 aluminum treated with Parker MP404 Lube & Parker PL303 Wash (treatment available from Parker Div. of Henkel Corp.) E - 1.6 mm thick 2024 T3 clad aluminum with surface prepared with an FPL etch The adhesive is mixed with about 1 % glass beads ("Microbead™ 1405 Class IV Engineering Grade" measuring between 0.35 to 0.246 mm in diameter, available from Cataphote, Inc.) to provide a 0.25 mm thick bond. The adhesive is then applied, within 30 minutes of mixing, to a 1.27 cm area on one end of one strip of aluminum and a second strip of aluminum is placed so that 1.27 cm of one end overlaps the adhesive and with the uncoated ends of each strip extending in opposing directions. The strips are clamped together and cured according to the conditions detailed in the examples. The prepared samples are conditioned for at least two hours at between 21 °C and 23 °C before testing to determine the initial (INIT) strength, and the aged samples are subjected to the aging conditions described below, and conditioned for 2 hours at between 21 °C and 23 °C before testing. Elevated temperature shear tests are run at the temperatures shown in the examples and samples are conditioned at the test temperature for at least 15 minutes, but no more than 30 minutes, before testing. The lap shear is determined using a tensile tester according to ASTM Test Method D 1002-72 under one of two conditions as follows: Shear Test I - The crosshead speed is run at the speed required to maintain a rate of loading between 800 to 1000 N/minute. Shear Test II - The crosshead speed is run at 1.27 cm/min. The lap shear is reported in units of megaPascals (MPa). The mode of failure is also recorded and noted as adhesive (A), wherein the adhesive pulls away from one of the aluminum strips, cohesive (C), wherein the adhesive splits leaving adhesive on each of the strips, or mixed (M), wherein both modes of failure are observed. If corrosion is visible along the edges of the aluminum strips or in extreme cases when there is an adhesive failure and corrosion is exposed, the approximate area of corrosion is also noted as a percentage of the total area covered by the adhesive. The test results represent the average of at least three independent samples involving a particular epoxy adhesive composition. AGING TESTS Samples are prepared and tested as described above for overlap shear except that the samples are aged in one or more of the following aging tests: 1) Water Soak The samples are soaked in 23 °C deionized water for 750 hours. 2) Salt Spray Test The samples are subjected to a 5% salt spray at 35 °C according to ASTM Bl 17-90 and tested after 750 hours. 3) Elevated Temperature/Humidity The samples are aged at 50°C and 95% RH for 750 hours. 4) Cyclic Corrosion Aging Test Lap shear samples are prepared and cured according to the above- described procedure. The sampls are then immersed into a 5 % NaCl colution at room temperature (betweeen about 21° and 23°) for 15 minutes. The samples are then drip dried at room temperature for 105 minutes and placed in a humidity chamber at 50° C, 90% relative humidity for 22 hours. Each immersion in the salt water solution marks the beginning of one cycle. On days when the samples are not immersed in water, the samples are stored in the heated humidity chamber detailed above and these days are not counted in the total number of cycles. If the samples are exposed to the above cyclic corrosion test under stress, six lap shear samples are prepared for each adhesive composition. The samples are loaded into a fixture which uses a compressed spring to exert a tensile load on the lap shear coupons. The spring is then compressed in a vise to the proper displacement to apply a tensile load of 2 MPa, 5 MPa, or 7 MPa, and the fixture is tightened to maintain the desired load. The samples are then exposed to the cyclic corrosion test conditions detailed above. The samples are checked daily for joint failure. Failure is noted when lap shear joints break apart. The failed sample is then replaced in the fixture with a solid strip of aluminum of the appropriate length and the cyclic corrosion test is continued until the third sample fails. Results are recorded as the number of days to failure (DAYS TO FAILURE). The remaining three samples are then tested for residual lap shear strength (RESIDUAL STRENGTH) according to the Lap Shear Test procedure described above. Results are recorded in MPa. Example 1 A one-part epoxy adhesive composition was prepared by mixing 10.73 parts of a methacrylate butadiene styrene terpolymer (Paraloid™EXL2691 available from Rohm & Haas) with 40.22 parts of a diglycidyl ether of bisphenol A (Epon™828 available from Shell Chemical Co.), and 13.4 parts of a flexible resin (CIBA™XB4122 made by Ciba Geigy), and heating at about 80 °C for about 60 minutes with constant stirring. The mixture was then cooled to about room temperature and the following were added and mixed with a high shear mixer: 2.68 parts of aluminum powder, 4.29 parts fumed silica (Cab-O- Sil™TS-720 silica available from Cabot Corp.), 5.36 parts barium metaborate (BUSAN 11-M2 available from Buckman Laboratories), 16.09 parts alumina tri-hydrate, 2.68 parts of calcium ion-exchanged silica gel (SHIELDEX™AC5 available from W. R. Grace & Co.), 3.21 parts dicyandiamide (Amicure CG 1200 available from Air Products, Inc.), and 1.34 parts hexakis (imidazole) nickel phthalate. The dicyandiamide and hexakis(imidazole) nickel phthalate were micronized to a particle size of about 10 micrometers. The adhesive composition was degassed, made into lap shear samples with substrate B, and cured for 40 minutes at 170 °C. The samples were tested for initial lap shear, and aged lap shear using Shear Test II. The samples were aged under the Cyclic Corrosion Exposure Test under stresses of 2 MPa, 5 MPa, and 7 MPa. Test results are shown in Table 1. TABLE 1 EX AGED SHEAR STRENGTH - MPa INITIAL 7 MPa STRESS 5 MPa STRESS 2 MPa STRESS STRENGTH DAYS TO RESIDUAL DAYS TO RESIDUAL DAYS TO RESIDUAL FAILURE STRENGTH FAILURE STRENGTH FAILURE STRENGTH 1 15.3 58-61-61 10.9 65-70-73 11.3 83* j^-p** Cl NT 46-47-48 NT NT NT NT NT * - No failure noted after 117 cycles ** - Not tested I The data in Table 1 show that bonds made with the adhesive composition of the invention last longer under a corrosive environment and stress than bonds made with a state-of-the-art adhesive containing a well known aluminum corrosion inhibition treatment. Comparative Example Cl A commercially accepted toughened epoxy adhesive composition having strontium chromate as a corrosion inhibitor was used. The strips were tested under a load of 7 MPa. Example 2 A one-part epoxy adhesive composition was prepared by mixing 41.7 parts EPIKOTE™828 (also sold as EPON™828), 16.6 parts of a diglyciyl ether of bisphenol F (EPIKOTE™862 available from Shell Chemical Co.), 13.2 parts Paraloid™EXL 2600 methacrylate butadiene styrene terpolymer (available from Rohm & Haas), and an adduct of diglycigyl ether of bisphenol A and carboxyl- terminated butadiene rubber (EPIREZ™58006 available from Rhone Poulenc) at about 80 °C for about an hour. The mixture was cooled to about room temperature and the following were added using a high shear mixer: 2.6 parts Aerosil™200 silica (available from DeGussa), 0.7 parts glycerol, 4 parts micronized dicyandiamide, 4 parts Ancamine™2014 S (available from Air Products and Chemical Co.), 0.7 parts glass beads having a particle size between about 90 - 150 micrometers (available from Glaverbel, of Belgium) and 3.3 parts Shieldex™AC5. The adhesive composition was degassed and tested for lap shear strength and aging on substrate E. The adhesives were cured for 120 minutes at 130°C under a heated platen at a pressure of 100 kiloPascals. Test results are shown in Table 2. Comparative Examples C2 - C3 Adhesive compositions were prepared as in Example 2 except that Example C2 had no corrosion inhibitor, and Example C3 had 3.3 parts of strontium chromate which is a state-of-the-art corrosion inhibitor. TABLE 2 EX ONE-PART ADHESIVE SHEAR STRENGTH - MPa / FAILURE MODE/ % CORROSION INITIAL 750 HRS/50°C, 750 HRS/23°C 750 HRS/35°C, 95% RH WATER IMMERSION 5% SALT SPRAY 23 °C 80°C 23 °C 80°C 23°C 80°C 23 °C 80°C 2 20.5/C/O 19.1/C/O 23.4/C/0 21.8/C/0 21.5/C/O 22.1/C/O 18.8/C/5 20.2/C/5 C2 20.8/C/O 17.2/C/O 21.8/C/0 20.3/C/O 21.6/C/O 20.3/C/O 14.4/M/20 4.2/M/30 I O C3 18.4/C/O 16.3/C/O 21.3/C/0 21.5/C/O 21.1/C/O 19.2/C/O 20.1/C/O 19.9/C/O I The data in Table 2 show that the use of a calcium ion-exchanged silica gel approximates the performance of strontium chromate as a corrosion inhibitor without the environmental hazards of heavy metals. Example 3 Part A of a 2-part adhesive composition was prepared by mixing 40 parts of a polyether diamine (sold by Minnesota Mining & Manufacturing Co. as Part A [amine curative] of a SCOTCHWELD™2216 BA Clear Amber epoxy adhesive kit), 6.0 parts 4,7, 10-trioxatridecane 1,13-diamine (TTD available from BASF), 8.0 parts 2,4,6-tri-dimethylaminomethyl phenol (K-54 available from Anchor Corp.), 3.0 parts amine-terminated butadiene rubber (ATBN 1300X16 available from B. F. Goodrich Co.), and heating to a temperature of about 70 °C to form a uniform mixture. The mixture was cooled to about room temperature, and 20 parts amorphous silicon dioxide (GP-71) and 3.0 parts fumed silica (Cab-O-Sil™ TS-720) were added with a high shear mixer. Part B of the 2-part epoxy adhesive composition was prepared by mixing together 15 parts of methacrylate butadiene styrene terpolymer (Paraloid™EXL2691) with 80 parts of a diglycidyl ether of bisphenol A (Epon™828) and 20 parts diglycidyl ether of cyclohexanedimethanol (Heloxy MK 107 made by Rhone Poulenc), and heating at about 80°C for about 60 minutes with constant stirring. The mixture was then cooled to about room temperature and the following were added and mixed with a high shear mixer: 2.0 parts Ca(SO 3 CF 3 ) 2 (micronized to a nominal particle size of about 10 micrometers), 2.5 parts epoxy silane (Z-6040 available from Dow Corning), 2.0 parts fumed silica (Cab-O-Sil™TS-720), 3.0 parts glass beads having an average diameter of about 0.01 inch [0.25mm] (available from Cataphote, Inc.), 20 parts amorphous silicon dioxide (GP-71 available from Harbison-Walker Corp.), and 18 parts glass bubbles (B37/2000 available from Minnesota Mining & Manufacturing Co.), and 5.0 parts of calcium ion-exchanged silica gel (SHIELDEX™AC5). An adhesive composition was prepared by mixing a 2: 1 volume ratio of Part B:Part A. The adhesive was made into lap shear samples as described above on substrates C and D at two cure conditions. Cure 1 indicates that the curing was done with a 6-second induction heating cycle to a temperature of 135 °C and then oven cured for 20 minutes at 170°C. Cure 2 indicates a room temperature (between about 21 °C to 23°) cure for 20 to 24 hours and a subsequent oven cure for 20 minutes at 170°C. The test was conducted using Shear Test 2. Test results are shown in Table 3. TABLE 3 SHEAR STRENGTH - MPa FAILURE MODE Example 3 INITIAL 10 CYCLES 20 CYCLES 30 CYCLES 1000 HOURS SALT SPRAY* Cure 1 /Substrate C 13.1/M 13.6/M 13.4/M 13.2/M 15.4/M Cure 2/Substrate C 10.7/M 11.9/M 11.7/A 12.0/A 3.7/M Cure 1 /Substrate D 17.0/M 15.9/M 16.6/M 15.7/M * *T** Cure 2/Substrate D 14.2/M 13.4/M 13.9/M 13.1/M jsjT** * Samples were exposed to continuous salt spray according to ASTM B-117-90 ** NT - NOT TESTED Example 4 Part A of a two-part epoxy adhesive composition was prepared by mixing 29 parts polyether diamine (TTD) and 5.0 parts of a 2,4,6-tri- dimethylaminomethyl phenol (DMP 30 available from Rohm & Haas), and applying sufficient heat to form a uniform solution. Part B of the 2-part epoxy adhesive composition was prepared by mixing together 20 parts of methacrylate butadiene styrene terpolymer (Paraloid™EXL2600) with 100 parts of a diglycidyl ether of bisphenol A (Epon™828) and heating at about 80 °C for about 60 minutes with constant stirring. The mixture was then cooled to about room temperature and the following were added and mixed with a high shear mixer: 4.0 parts fumed silica (Aerosil™ R202 available from DeGussa), 2.5 parts glass beads having a particle size between about 90 - 150 micrometers (available from Glaverbel), and 5.0 parts of calcium ion-exchanged silica gel (SHIELDEX™AC5 available from W. R. Grace & Co.). A 2-part adhesive was prepared by mixing Part B with Part A (in a 2: 1 volume ratio and Part B:Part A) and preparing lap shear samples on substrate E. The adhesive was cured for 24 hours at 23 °C under 100 kPa pressure and then oven cured for 60 minutes at 80° C. The samples were tested for initial lap shear and aged lap shear strength using Shear Test 1. Data is shown in Table 4. Comparative Examples C4 - C5 Two-part epoxy adhesive compositions were prepared as in Example 4 except Example C4 had no corrosion inhibitor, and Example C5 had 5 parts of strontium chromate. Samples were tested as in Example 4 and test results are shown in Table 4. TABLE 4 EX TWO-PART ADHESIVE SHEAR STRENGTH - MPa / FAILURE MODE/ % CORROSION INITIAL 750 HRS/50°C, 750 HRS/23°C 750 HRS/35°C, 95% RH WATER IMMERSION 5% SALT SPRAY 23 °C 80°C 23 °C 80°C 23°C 80°C 23 °C 80°C 4 36.1/C/O 31.8/C/0 32.9/C/0 28.1/C/0 35.4/C/O 28.5/C/0 32.6/M/5 15.1/M/10 I I C4 35.9/M/O 30.9/M/O 34.9/M/O 28.1/M/O 34.1/M/O 27.2/M/0 10.9/M/30 2.3/M/30 C5 36.9/C/O 31.4/C/O 32.0/C/0 28.0/C/0 34.8/C/0 30.4/C/O 32.6/C/0 30.3/C/0 Example 5 Part A of a 2-part adhesive composition was prepared by mixing 40 parts of a polyether diamine (sold by Minnesota Mining & Manufacturing as Part A of SCOTCHWELD™2216 BA Clear Amber epoxy adhesive kit), 6.0 parts polyether diamine (H221 available from Union Carbide Inc.), 8.0 parts 2,4,6-tri dimethylaminomethyl phenol (K-54) 3.0 parts-amine terminated butadiene rubber (ATBN 1300X16 available from B. F. Goodrich Co.), and 5 parts imidazole and heating to about 70 °C with constant stirring to form a uniform mixture. The mixture was cooled to about room temperature and 20 parts amorphous silicon dioxide (GP-71) and 3.0 parts fumed silica (Cab-O- Sil™TS-720) with a high shear mixer. Part B of the composition was prepared as in Example 3 except that 19.5 parts of glass bubbles were used. An adhesive composition was prepared by mixing 2 parts of Part B to one part of Part A by volume.

Claims: WHAT IS CLAIMED IS: 1. A curable, structural epoxy adhesive composition comprising: (a) an epoxy resin having an average epoxide functionality of greater than one; (b) a base curing agent in an amount sufficient to cure the epoxy resin through reaction of a nucleophilic or electrophilic group contained in the base curing agent with the epoxy ring contained in the epoxy resin; (c) particles comprising an inorganic oxide of silica or alumina having cations bound thereto, the particles being of a type which and present in an amount effective to inhibit or prevent corrosion of metal to which the epoxy adhesive composition has been applied. 2. A composition according to Claim 1, wherein the cation is selected from the group consisting of Ca 2+ , Zn 2+ , Co 2"1" , Pb 2+ , Sr 2"1" , Li + , Ba 2+ and • Mg 2+ . 3. A composition according to Claim 1, wherein the cation is selected 0 from the group consisting of Ca 2+ and Zn 2+ . 4. A composition according to Claim 1, wherein the particles comprise a calcium ion-exchanged amorphous silica gel. 5 5. A composition according to Claim 1, further comprising a toughening agent having an epoxide compatible component substantially soluble in the epoxy resin and an epoxide incompatible component substantially insoluble in the epoxy resin. 0 6. A composition according to Claim 1, further comprising an accelerator to increase the cure rate of the epoxy adhesive composition. 7. A method for inhibiting or preventing corrosion of a metal substrate which has been bonded to another substrate, the method comprising including particles comprising an inorganic oxide of silica or alumina having cations bound thereto in the adhesive composition being employed to bond the metal substrate to the other substrate, the particles being of a type which and present in the adhesive composition in an amount effective to inhibit or prevent corrosion of the metal substrate in the area thereof which is in contact with the adhesive composition. 8. A method according to Claim 7, wherein the other substrate is also metal.

Inventors: LAMON ALAIN H, LAMON, ALAIN, H.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1994
Main classification: C09J163
IPC codes: C09J11, C09J163